Bleaching cellulosic pulps with gaseous chlorine at a pH of 7 to 10 in a multistage bleaching process

ABSTRACT

This invention relates to multistage bleaching of cellulosic pulp wherein gaseous chlorine is contacted with cellulosic pulp at high consistency in the third or subsequent stage of a multistage bleaching process, while maintaining the pH of the pulp between about 7 and about 10.

United States Patent 1191 Christiansen et al.

[ Sept. 23, 1975 BLEACHING CELLULOSIC PULPS WITH GASEOUS CHLORINE AT A pH OF 7 TO 10 IN A MULTISTAGE BLEACHING PROCESS [75] Inventors: Claud B. Christiansen; William R.

Dial, both of Akron, Ohio [73] Assignee: PPG Industries, Inc., Pittsburgh, Pa.

[22] Filed: Oct. 23, 1973 [21] Appl. No.: 408,770

[52] US. Cl. 162/66; 162/88; 162/89 [51] Int. Cl. D21C 9/12 [58] Field of Search 162/89, 66, 88

[56] References Cited UNITED STATES PATENTS 2,968,589 1/1961 Saunders et a]. 162/89 X 3,433,702 3/1969 Jack 8! a1 162/88 X 3,501.374 3/1970 Jack et al 162/89 3579419 5/1971 Yorston et a1. 162/66 3,586,599 6/1971 Yorston 61 al. 162/88 x 3,619,349 11/1971 Liebergott et al. 162/66 3,630,828 12/1971 Liebergott 81 al. 162/66 x Primary Examiner-S. Leon Bashore Assistant ExainirierArthur L. Corbin Attorney, Agent, or FirmMark Levin 17 Claims, N0 Drawings BLEACHING CELLULOSIC PULPS WITH 7 GASEOUS CHLORINE AT A PI-I OF 7 TO 10 IN A MULTISTAGE BLEACHING PROCESS STATE OF THE ART In the multistage bleaching of an unbleached chemical cellulosic pulp, i.e. wood pulp, the pulp from the digestion stage in, for example, the kraft, soda, or sulfite process, normally after washing, is treated to render at in water or soluble in dilute aqueous alkali, such as dilute aqueous solutions of sodium hydroxide, sodium sulfite or ammonium hydroxide. To accomplish this solubilization of ligneous impurities, the pulp is conventionally treated with agents such as chlorine, chlorine dioxide, or the oxides of nitrogen, more usually chlorine or chlorine dioxide. This process which is more a purification step or pretreatment step rather than a true bleaching step is generally considered the first stage in a multistage pulp bleaching process.

After the first stage treatment, the pulp, after washing with water, is subjected to alkaline extraction with, for example, sodium hydroxide to remove water insoluble ligneous impurities. The extraction is generally conducted on the pulp at low consistency in the range of 10 to 14%, a temperature range of 40 to 70C. and at a retention time of about 1 to 2 hours. This sequence is generally considered the second stage of a multistage bleaching process.

Subsequent to the basic extraction stage and normally after water washing, the pulp is subjected to a bleaching or oxidizing stage, which is normally considered the third stage in a multistage bleaching process.

The preceding three stages are present in essentially all multistage bleaching processes for chemical cellulosic pulps although variations are possible. For example, with pulp made from certain wood species, treatment with sodium or calcium hypochlorite may be desirable between the stage two extraction and third stage oxidative or bleach treatment. Further, with certain pulps, especially where high brightness is desired, there is employed an additional fifth bleaching or oxidizing stage which is not infrequently similar to the third stage. These two oxidizing or bleaching stages are separated by a fourth stage, a basic extraction procedure similar to the second stage. This five stage bleaching process is commonly utilized with kraft pulps.

The use of chlorine in various forms in the treatment of chemical pulps has been described in the art.

Canadian Pat. No. 886,479 describes treating high consistency pulps with high concentrations of chlorine gas or gaseous chlorine dioxide mixtures for relatively short times. The treatment may be conducted in a chlorination, extraction, chlorination, extraction sequence.

U.S. Pat No. 3,433,702 generally describes the use of aqueous or gaseous chlorine in various stages of pulp bleaching and is drawn to a process wherein low consistency pulps are bleached in a sequence wherein pulp is first reacted with chlorine dioxide until the chlorine dioxide is at least partially depleted. The pulp is then reacted with chlorine.

US. Pat. No. 3,707,436 describes a process wherein pulp fiber is exploded or expanded. An aqueous solu tion of the fiber is then subjected to two chlorination stages wherein aqueous or gaseous chlorine may be employed.

least a portion of the ligneous impurities readily soluble US. Pat, No. 3,560,330 describes bleaching low consistency pulps with chlorine with relatively long chlorination times.

US. Pat. No. 2,166,330 describes the use of mixtures of chlorine and chlorine dioxide to avoid fiber degradation.

US. Pat. No. 3,619,349 describes bleaching high consistency pulps with gaseous chlorine monoxide in a multistage bleaching process.

DESCRIPTION OF THE INVENTION It has now been found that significant benefits can be obtained in bleaching a chemical cellulosic pulp, by employing a novel bleaching step in at least one stage subsequent to the initial steps of solubilizing ligneous impurities and followed by extraction with a base, in a multistage bleaching process.

The novel process step comprises accomplishing bleaching of the resultant fibrous cellulosic pulp in a third or subsequent step in the multistage bleaching process by contacting the pulp at a high consistency and in the form of fluffed or shredded fibers and fiber aggregates with gaseous chlorine.

In particular, it has been found that in the bleaching of chemical cellulosic pulps in the gaseous phase at high consistency and in fluffed and shredded form, chlorine can be used with advantage in those stages of a multistage bleaching process where chlorine dioxide or chlorine monoxide have been used to achieve a simi lar oxidative bleaching effect.

The use of gaseous chlorine presents several distinct advantages, the first being safety. Both chlorine dioxide and chlorine monoxide tend to form explosivemixtures which create safety hazards. The handling of chlorine presents a significantly lower safety hazard in this re-. gard. Cost is another significant factor, both the use of chlorine dioxide and chlorine monoxide require facilities for their preparation, and which thereof add to the cost of the process and further which can introduce variables into their use.

Further, the use of gaseous chlorine presents significant time and temperature advantages over the use of chlorine dioxide. It has been found that in the bleaching of a chemical cellulosic pulp, particularly wood pulp, such as a kraft or sulfite pulp, at high consistency by contacting with gaseous chlorine, it is possible to achieve substantial improvements in Elrepho brightness within 1 to 5 minutes where a typical gaseous chlorine dioxide treatment requires 30 minutes.

In a particularly preferred embodiment, gaseous chlorine is employed in a highconsistency. bleaching stage subsequent to an intermediate bleaching stage employing chlorine dioxide; for example, in the fifth stage of a process which involves subjecting chemical pulp to chlorination extraction, chlorine dioxide, extraction, with a fifth stage being the contacting of the resultant pulp at high consistency with gaseous chlorine as described herein.

The process of the invention is equally applicable to pretreated "or prebleached pulps having a relatively low Permanganate Number (TAPPI Standard T-2 14), which are items of commerce. These are chemical cellulosic pulps which have been subjected to pretreatment or prebleaching, for example, by subjection to the first two stages of a conventional multistage bleaching process, and havehad at least a significant portion of their ligneous constituents removed. For example,thc' process of the invention is useful in bleaching virtually 3 any partially bleached chemical cellulosic pulp preferably having a Permanganate Number less than about 5 and preferably less than above 3.

The pulp contacted with gaseous chlorine in the process of the invention is a high consistency pulp, that is a pulp which is essentially free of a continuous aqueous phase. Generally, high consistency pulps contain at least about fibers on a moisture-free fiber basis. Preferably, the pulps employed have a consistency of about 15 to about 50%. It is desirable that the pulp have a consistency of about 15 to about 35%. The primary criteria for distinguishing high consistency pulp from'low consistency pulp, that is pulp which is a slurry of fibers in a discrete aqueous phase, is the substantial absence of a discrete liquid aqueous phase. Otherwise stated, in high consistency pulp substantially all water present is retained by the fibers so that there can be created Ia shredded and fluffed material which has a large surface area in contact with the surrounding atmosphere.

In the process of the invention, gaseous chlorine is fed into a bleaching zone containing cellulosic pulp at high consistency to thereby induce bleaching. This bleaching zone may take the form of a batch reactor or a continuous reactor where the pulp and chlorine are charged in either a countercurrent or concurrent fashion with means to provide adequate contacting of the charged materials. The mechanism of bleaching is not fully know-n and while gaseous chlorine is fed into the bleaching zone, it may, in fact, be transformed, for example, at the gas-pulp interface, or in or on the fiber, at least in part, to at least one other chemical moiety which may, in fact, be the actual bleaching moiety.

The fluffed high consistency pulp can be contacted with a gaseous feed which is essentially 100% chlorine, devoid of intentionally added amounts of chlorine monoxide, for example, i.e. essentially pure chlorine gas, or the chlorine may be diluted with a non-reactive gas such as air, carbon dioxide or nitrogen, to provide a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine. Typically, when a non-reactive gas diluent is employed, the chlorine is diluted with such a gas in the range of about 4:1 to 1:4. The use of a pure chlorine-containing gas is in contrast to the use of chlorine monoxide or chlorine dioxide which is diluted for safe usage.

To aid in the distribution of the chlorine throughout the pulp, it is'preferred that the pulp be agitated during or after the introduction of the desired amount of chlorine into the bleaching zone. This agitation may be accomplished by tumbling or other appropriate means.

The amountof chlorine added is subject to wide variation and obviously the history of the pulp, including the length and severity of prior treatments, governs the amount of chlorine necessary to achieve optimum results. Surprisingly, it has been found, however, that the introduction of low amounts of chlorine below 2% by weight of chlorine based on moisture-free fiber into the bleaching zone accomplishes substantial bleaching. Typically, the use of chlorine in an amount of about 1% by weight or less based on the moisture-free fiber provided highly effective bleaching, such as that achieved by the use of chlorine dioxide or chlorine monoxide. While the use of higher amounts of chlorine provide effective bleaching, when higher amounts of chlorine are employed, fiber viscosities tend to be lowered.

The temperature at the time of contact between the pulp and the gaseous chlorine is not particularly critical in achieving a bleaching effect. It is preferred that the temperature in the contacting zone be at about room temperature. Bleaching is rapidly accomplished at ambient temperature and, while elevated temperature can be employed, heating is unnecessary and the use of elevated temperatures tend to reduce fiber viscosity.

The pH maintained in the process has a significant effect on the viscosity of the pulp obtained. It has been found desirable to maintain the pH of the pulp at a pH which is at least about neutral or alkaline during the process of the invention. Typically at the end of the gaseous chlorine contacting step, as measured as hereinafter described, is between about 7 and about 10 and preferably between about 7 and about 9. The pH of the reaction mixture may be positvely controlled by several means. Since the pulp treated by the process of the invention has been previously subjected to alkaline extraction, the amount of subsequent washing may be controlled so that sufficient base remains in the fibers to maintain the pH in the desired range throughout the gaseous chlorine contacting step. Another method of controlling the pH comprises adding to the pulp, prior to contacting the pulp with gaseous chlorine or incrementally during the contacting of the pulp with gaseous chlorine, sufficient base to maintain the pH in the desired range through the gaseous chlorine contacting step. The base employed can vary widely since its primary function is pH maintenance. Strong bases, such as sodium hydroxide and the like, may be employed. Relatively weak bases, such as ammonium hydroxide and the like, are equally operable. Where it is desirable to maintain the highest possible fiber viscosities, it has been found desirable to employ a bufferingagent, such as an alkali metal salt of a weak acid; for example, sodium carbonate, may be employed. The alkali metal carbonates, such as sodium carbonate, sodium bicarbonate, either hydrous or anhydrous, and the like, are highly useful.

Since the direct measurement of the pH of the high consistency pulp is difficult, since essentially no free aqueous phase is present, pH is measured by diluting the high consistency pulp with water to form a low consistency pulp with a consistency of about 7%, and measuring the pH of the aqueous phase so produced.

The reaction time necessary to achieve substantial bleaching effect is relatively short. The substantial portion of the bleaching effect occurs in the first 1 to 5 minutes of contact time between the high consistency fluffed pulp and the gaseous chlorine, while contact may be continued for up to 30 minutes or even an hour, such time periods are generally not necessary and may contribute to fiber viscosity degradation.

' .The invention is further described in the following examples which are to be considered illustrative rather than limiting. All parts and percentages are by weight unless otherwise specified.

' EXAMPLE 1 tency by contacting the pulp with chlorine water 5. (5.25% Cl on moisture-free fiber) at room temperature for 60 minutes. The. chlorine water was then drained and the pulp thoroughly washed with water.

The chlorination stepwas followed by a second stage extraction step wherein the pulp at 12% consistency was contacted with 10% sodium hydroxide (3% sodium hydroxide on moisture-free fiber) at 165 F. for 90 minutes. The caustic solution was drained and the pulp thoroughly washed with water.

The resultant pulp at 12% consistency was then bleached with chlorine dioxide, by heating the pulp to 165F. and adding 0.5% chlorine dioxide (7 gm/liter solution in water) and holding the mixture at 165F. for 2 hours, drained and thoroughly washed with water.

The resultant pulp was then subjected to a fourth stage extraction step wherein the pulp at 12% consis- 6 cupying about half the volume of the chamber. Chlorine gas diluted with nitrogen (3 volumes of N /volume of C1 was passed into the bottom of the chamber and was absorbed within the fiber mass. The chamber was sealed and repeatedly (10-15 times) rapidly inverted to assure mixing by tumbling of the fibers. The chamber was then allowed to stand the remainder of the reaction time.

At the end' of the reaction time, the contents of the chamber were placed in sufficient water to reduce the consistency to 7.5%, at which time, after mixing, pH was measured by squeezing the pulp in a muslin cloth to remove water and measuring the pH of the water.

The pulp was then washed with water at 120F. and filtered. The results of the multistage treatment are set forth in Table I.

TABLE I Bleaching of Prebleached (CEDE) Kraft Pulp with Chlorine Softwood Kraft Pulp: Unbleached K No. 14.9; CED Viscosity 18.4 cp

Consistency, CED Run Chemical, m.f. Pulp m.f. Temp, Time, pH Viscosity, Elrepho No. Cl NaOl-l Na CQ, Pulp F. Min. Final cp Brightness Prebleached Kraft Pulp 15.0 82.8 1 0.3. 0.10 30 80 4.2 v1 1.8 89.5 2 0.3 0.15 30 8O 5 4.5 11.9 89.9 3 0.3 0.20 30 '80 5 5.7 12.5 89.9 4 0.3 0.40 30 80 5 7.7 12.0 91.1 5 0.3 0.20 30 80 5 4.4 12.0 89.4 6 0.3 0.25 30 80 5 5.4 12.5 89.8 7 0.3 0.30 30 80 5 6.7 12.6 89.1 8 0.3 0.40 30V 80 5 7.0 12.8 90.3 9 0.3 0.60 30 80 5 g 7.8 13.0 90.5 10 0.17 0.20 30 80 5 7.1 13.3 90.8 1 1 0.17 0.25 30 80 5 7.3 13.4 90.9 12 0.17 0.30 30 80 5 7.6 13.3 90.7 13 0.3 0.10 30 80 1 7.5 1 1.6 90.9 14 0.3 0.15 30 80 l 7.6 12.0 90.6 15 0.3 0.20 80 1 7.9 12.4 90.2 16 0.30 0.3 30 80 l 6.9 12.1 90.9 17 0.30 0.4 30 80 1 7.3 12.2 91.1 18 0.30 0.5 30 8O 1 8.4 12.7 90.7

When aliquots of the pretreated pulp, as formed in the first four stages, fluffed and treated, as set forth above, were subjected to a fifth stage bleaching under the same conditions, but replacing chlorine with an equivalent amount of chlorine monoxide, the following results were obtained, as set forth in Table I1:

TABLE II Bleaching of Prebleached (CEDE) Kraft Pulp with Chlorine Monoxide Consistency, CED Run Chemical, 7c m.f. Pulp m.f. Temp., Time, pH Viscosity. Elrepho No Equiv. C1 NaOH Na CO Pulp F. Min. Final cp Brightness 0.3% available chlorine is equivalent to 0.18% C1 0 0.17% available chlorine is equivalent to 0.107: C1 0 which were treated in the followin manner. The org p EXAMPLE 2 tions were sprayed with water sufficient to reduce the consistency to 30%. Where base or buffers were added to adjust pH (i.e. NaOH or Na CO these were incorporated into the water sprayed upon the fluffed fibers. The fluffed fibers were placed into a glass chamber oc- 7 8 The pulp was treated by a first stage chlorination and a into a glass chamber, occupying about half the volume second stage caustic extraction to yield a pulp having a of the chamber. The'fibe'rs are rested upon a perforated CED viscosity of 14.0 centipoise and an Elrepho glass grid which raises thethe fibers above the gas inlet brightness of 48.4. The third stage chlorination step in the bottom of the chamber to allow for gas circulawas conducted as in Example 1. The results found in tion about the fibers. Chlorine gas diluted with nitrogen treating divided portions of the extracted high consi's- (3" volumes of N /volume of C1 was passed into the tency'fluffed pulp are summarized in Table III. bottom of the chamber and absorbed within the fiber TABLE 111 Bleaching of Pretreated (CE) Softwood Kraft Pulp with Chlorine Softwood Kraft Pulpi Unbleached K No. 14.9; CED Viscosity l8.4 cp

Consistency, I CED Run Chemical, m.f. Pulp m.f. Temp, Time. pH Viscosity, Elrepho No. Cl NaOH Pulp F. Min. Final cp Brightness Pretreated Kraft Pulp 14.0 48.4 1 0.6 O 30 80 5 3.2 12.6 72.5 2 0.6 0.20 30 80 5 3.9 10.9 74.3 3 0.6 0.40 30 8O 5 4.9 10.3 75.0

The'pretreated pulp contained caustic soda left in the pulp by means of a final wash with caustic solution to obtain good distribution of buffer before fluffing.

mass. The chamber was sealedand repeatedly inverted EXAMPLE 3 to tumble the fibers. The chamber was then allowed to This example shows the use of gaseous chlorine to stand for the remainder of the reaction time. bleach high consistency pulp in the third and fifth The above bleaching step was followed by a fourth stages of a multistage bleaching process. The pulp stage extraction step where the fibers were extracted treated was a softwood kraft pulp having an unwith dilute caustic (1.0% caustic on a moisture-free bleached K'number of 14.9 and a CED viscosity of 15.4 fiber basis), at 12% consistency, at 165F. for 90 mincentipoise. utes. The fibers were then thoroughly washed with wa- The pulp was pretreated, in a conventional manner, ter.

with a first stage chlorination step at 3% consistency by The fibers were then dried to above 70% consistency contacting chlorine water (5% chlorine on a moistureand treated in a fifth stage with gaseous chlorine, at

free pulp basis) at room temperature (75F.) for 60 30% consistency in the same manner as the third stage.

minutes. The chlorine water was then drained and the The conditions and results of each of the stages, with pulp thoroughly washed with water. varying conditions in the third and fifth stage gaseous The chlorination step was followed by a second stage chlorination steps, are set forth in Table IV.

TABLE IV Bleaching Softwood Kraft Pulp by Sequence CEC,,EC,,

Softwood Kraft Pulp: Unbleached K No. 14.9; CED Viscosity 15.4 cp

I Consistency, CED Run Stage Chemical, m.f. Pulp m.f. Temp., Time, pl-l Viscosity, Elrepho No. No'. Cl NaOH Pulp F. Min. Final cp Brightness extraction step wherein the pulp was contacted at 12% The conditions and quantities of reagents in the consistency with 10% sodium hydroxide (3% sodium above examples may be varied within the scope of the hydroxide on a moisture-free fiber basis) at 165F. for disclosure while obtaining results within the scope of 75 minutes. The caustic solution was drained and the 60 this invention. Likewise, various pulps or reagents, pulp thoroughly washed with water. such as those described hereinabove, may be substi- The resultant pulp dried to about 70% consistency tuted for those exemplified to obtain equivalent results. was then shredded and fluffed in increments in a War- According to the provisions of the Patent Statutes, ing Blendor. The pulp was divided into portions which there are described above the invention and what are weretreated, in a third stage, in the following manner. 65 now considered to be its best embodiments. However, The portions were sprayed with a fine mist of water within the scope of the appended claims, it is to be uncontaining sufficient sodium hydroxide to obtain the derstood that the'invention may be practiced otherwise final desired pH. The fluffed fibers were then .placed than specifically described.

What is claimed is:

least to a chemicaltreatment stage to solubilize ligneous impurities, followed by a-subs'equent alkalineextraction treatment, the improvement which comprises contacting the resultant pulp in at least one subsequent stage at high consistency and in the form of fluffed or shredded fibers or fiber aggregates, with a gaseous medium wherein the active bleaching contacting com o, nent consists essentially of gaseous chlorine wherein the amount of gaseous chlorine in each gaseous chlorine contacting stage is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp and the pH of the pulp is maintained between about 7 and about 10 during the gaseous chlorine contacting stage, thereby effecting a bleaching of said pulp.

2. In a multistage process for bleaching a fibrous cellulosic pulp wherein the pulp has been subjected at least to a chemical treatment stage to solubilize ligneous impurities, followed by a subsequent alkaline extraction treatment, the improvement which comprises contacting the resultant pulp in at least one subsequent stage at high consistency and in the form of fluffed or shredded fibers or fiber. aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine in each gaseous chlorine contacting stage is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp, thereby effecting bleaching of said pulp, where the pulp prior to contacting with chlorine has been subjected to an alkaline extraction stage wherein the amount of residual base in the pulp at the termination of the extraction stage is controlled to provide a pH of between about 7 and about 10 at the termination of the gaseous chlorine contacting stage.

3. In a multistage process for bleaching a fibrous cellulosic pulp wherein the pulp has been subjected at least to a chemical treatment stage to solubilize ligneous impurities, followed by a subsequent alkaline extraction treatment, the improvement which comprises contacting the resultant pulp in at least one subsequent stage at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine in each gaseous chlorine contacting stage is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp, thereby effecting bleaching of said pulp, sufficient base being added to the gaseous chlorine contacting stage to maintain the pH in the gaseous chlorine contacting stage between about 7 and about 10.

essentially=of gaseous chlorine wherein theamount of gaseouschlorine-in each gaseous chlorine contacting stage is an effective bleaching .inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp,,thereby effecting bleaching of said pulp, sufficient amounts of an alkali metal carbonate being added to the gaseous chlorine contacting stage to maintain the pH in the gaseous chlorinecontacting stage between about 7 and. 10.

6. A process, as in claim 3, wherein the base is sodium hydroxide. i I 1. 1

7. A process, as in claim 1, wherein'the high consistency pulp has a consistencybetween about 15 and about 50. j

8. In a process of bleaching a cellulosic pulp which comprises:

a. treating a chemical cellulosic pulp in a first stage to solubilize ligneous impurities,

b. extracting the resultant pulp with aqueous base,

c. bleaching the resultant pulp with chlorine dioxide,

and

d. extracting the resultant pulp with aqueous base,

the improvement which comprises,

e. contacting the resultant pulp at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine is an effective bleaching inducing amount of chlorine which is about 1% or less by.

10. A process, as in claim 8, wherein a pH maintain:

ing amount of base is added to the pulp prior to the gaseous chlorine contacting stage.

11. A process, as in claim 10, wherein the base is an alkali metal salt of a weak acid. j

12. A process, as in claim 11, whereinthe base is carbonate of sodium. 1

13. A process, as in claim 10, wherein the base is sodium hydroxide.

14. A process, as inclaim 8, wherein the high consistency pulp has a consistency between about 15.- and about 50. a

15. In a process of bleaching a cellulosic pulp which comprises: l

a. treating a chemical cellulosic pulp to solubilize ligneous impurities, and

b. extracting the resultant pulp with aqueous base,

the improvement which comprises,

c. contacting the resultant pulp at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists. essentially of gaseous chlorine and the pH of the pulp is maintained between about 7 and about 10 during the gaseous chlorine contacting stage, thereby effecting bleaching of said pulp,

d. extracting the resultant pulp with aqueous base,

e. contacting the resultant pulp at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with -a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine is an effective bleaching inducing amount of chlorine which is about 1% or less by 1 weight based on the weight of moisture-free cellulosic pulp, thereby effecting bleaching of said pulp. 16. A process for bleaching a pretreated or prebleached and extracted fibrous cellulosic pulp having a Permangan'ate Number of about 5 or less which comprises contacting said pulp at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of'gaseous chlorine wherein the amount of gaseous chlorine is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp and the pH of the pulp is maintained between'about 7 and about 10 during the gaseous chlorine contacting, thereby effecting a bleaching of said pulp.

' 17. A process, as in claim 16, wherein the high consistency pulp has a consistency between about 15 and about 50. 

1. IN A MULTISTAGE PROCESS FOR BLEACHING A FIBROUS CELLULOSIC PULP WHEREIN THE PULP HAS BEEN SUBJECTED AT LEAST A CHEMICAL TREATMENT STAGE TO SOLUBILIZE LIGNEOUS IMPURITIES, FOLLOWED BY A SUBSEQUENT ALKALINE EXTRACTION TREATMENT, THE IMPROVEMENT WHICH COMPRISES CONTACTING THE RESULTANT PULP IN AT LEAST ONE SUBSEQUENT STAGE AT HIGH CONSISTENCY AND IN THE FORM OF FLUFFED OR SHREDDED FIBERSS OR FIBER CONSISTENCY AND IN THE FORM MEDIUM WHEREIN THE ACTIVE BLEACHING CONTACTING COMPONENT CONSISTS ESSENTIALLY OF GASEOUS CHLORINE WHEREIN THE AMOUNT OF GASEOUS CHLORINE IN EACH GASEOUS CHLORINE CONTACTING STAGE IS AN EFFECTIVE BLEACHING INDUCING AMOUNT OF CHLORINE WHICH IS ABOUT 1% OR LESS BY WEIGHT BASED ON THE WEIGHT OF MOISTUREFREE CELLULOSIC PULP AND THE PH OF THE PULP IS MAINTAINED BETWEEN ABOUT 7 AND ABOUT 10 DURING THE GASEOUS CHLORINE CONTACTING STAGE, THEREBY EFFECTING A BLEACHING OF SAID PULP.
 2. In a multistage process for bleaching a fibrous cellulosic pulp wherein the pulp has been subjected At least to a chemical treatment stage to solubilize ligneous impurities, followed by a subsequent alkaline extraction treatment, the improvement which comprises contacting the resultant pulp in at least one subsequent stage at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine in each gaseous chlorine contacting stage is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp, thereby effecting bleaching of said pulp, where the pulp prior to contacting with chlorine has been subjected to an alkaline extraction stage wherein the amount of residual base in the pulp at the termination of the extraction stage is controlled to provide a pH of between about 7 and about 10 at the termination of the gaseous chlorine contacting stage.
 3. In a multistage process for bleaching a fibrous cellulosic pulp wherein the pulp has been subjected at least to a chemical treatment stage to solubilize ligneous impurities, followed by a subsequent alkaline extraction treatment, the improvement which comprises contacting the resultant pulp in at least one subsequent stage at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine in each gaseous chlorine contacting stage is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp, thereby effecting bleaching of said pulp, sufficient base being added to the gaseous chlorine contacting stage to maintain the pH in the gaseous chlorine contacting stage between about 7 and about
 10. 4. A process, as in claim 3, wherein the base is an alkali metal salt of a weak acid.
 5. In a multistage process for bleaching a fibrous cellulosic pulp wherein the pulp has been subjected at least to a chemical treatment stage to solubilize ligneous impurities, followed by a subsequent alkaline extraction treatment, the improvement which comprises contacting the resultant pulp in at least one subsequent stage at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine in each gaseous chlorine contacting stage is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp, thereby effecting bleaching of said pulp, sufficient amounts of an alkali metal carbonate being added to the gaseous chlorine contacting stage to maintain the pH in the gaseous chlorine contacting stage between about 7 and
 10. 6. A process, as in claim 3, wherein the base is sodium hydroxide.
 7. A process, as in claim 1, wherein the high consistency pulp has a consistency between about 15 and about
 50. 8. In a process of bleaching a cellulosic pulp which comprises: a. treating a chemical cellulosic pulp in a first stage to solubilize ligneous impurities, b. extracting the resultant pulp with aqueous base, c. bleaching the resultant pulp with chlorine dioxide, and d. extracting the resultant pulp with aqueous base, the improvement which comprises, e. contacting the resultant pulp at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp, whilE the pH of the pulp is maintained between about 7 and about 10 during the gaseous chlorine contacting stage, thereby effecting bleaching of said pulp.
 9. A process, as in claim 8, wherein the pulp prior to contact with chlorine has been subjected to a basic extraction stage and wherein the amount of residual base in the pulp at the termination of the extraction stage is controlled to provide the desired pH in the gaseous chlorine contacting stage.
 10. A process, as in claim 8, wherein a pH maintaining amount of base is added to the pulp prior to the gaseous chlorine contacting stage.
 11. A process, as in claim 10, wherein the base is an alkali metal salt of a weak acid.
 12. A process, as in claim 11, wherein the base is carbonate of sodium.
 13. A process, as in claim 10, wherein the base is sodium hydroxide.
 14. A process, as in claim 8, wherein the high consistency pulp has a consistency between about 15 and about
 50. 15. In a process of bleaching a cellulosic pulp which comprises: a. treating a chemical cellulosic pulp to solubilize ligneous impurities, and b. extracting the resultant pulp with aqueous base, the improvement which comprises, c. contacting the resultant pulp at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine and the pH of the pulp is maintained between about 7 and about 10 during the gaseous chlorine contacting stage, thereby effecting bleaching of said pulp, d. extracting the resultant pulp with aqueous base, e. contacting the resultant pulp at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp, thereby effecting bleaching of said pulp.
 16. A process for bleaching a pretreated or prebleached and extracted fibrous cellulosic pulp having a Permanganate Number of about 5 or less which comprises contacting said pulp at high consistency and in the form of fluffed or shredded fibers or fiber aggregates with a gaseous medium wherein the active contacting component consists essentially of gaseous chlorine wherein the amount of gaseous chlorine is an effective bleaching inducing amount of chlorine which is about 1% or less by weight based on the weight of moisture-free cellulosic pulp and the pH of the pulp is maintained between about 7 and about 10 during the gaseous chlorine contacting, thereby effecting a bleaching of said pulp.
 17. A process, as in claim 16, wherein the high consistency pulp has a consistency between about 15 and about
 50. 